The invention relates to a disk brake disc.
In particular, the present invention relates to a disk-brake disk comprising a braking band for cooperating with brake calipers in order to exert a braking effect on a vehicle. The braking band comprises a body which extends about an axis of symmetry and is defined laterally by braking surfaces. The body can be produced by causing a mixture comprising bundles of filaments constituted substantially by carbon to interact with silicon at a temperature high enough to bring about fusion of the silicon.
The term xe2x80x9cfilaments constituted substantially by carbon xe2x80x9d may include fibrous materials produced by pyrolysis of various products of synthetic origin, for example, polyacrylonitrile (PAN) or polysilazane, or of natural origin, for example, pitches or cellulose-based natural sources such as vegetable fibres and wood.
The term xe2x80x9cbundles of filaments xe2x80x9d may include groups of filaments variable from 3000 to 50000 units and having diameters of between 2 and 3 mm, associated with one another and impregnated with a resin, for example, polyurethane resin. The bundles are then broken up so as to have lengths of less than 30 mm and, finally, are arranged randomly in the mixture.
These randomly-arranged bundles of filaments are commonly defined on the basis of the number of units constituting the bundle, for example, 3K, 10K, 50K, etc.
Composite ceramic materials are used in various applications which require good impact strength, compression strength and resistance to heat generated by friction, which characteristics cannot be ensured by purely ceramic materials because of their intrinsic fragility.
In particular, known composite ceramic materials for braking applications are produced by the interaction of silicon with a mixture comprising bundles of carbon filaments and additives at a temperature at which the silicon is in the fused state.
Bundles of filaments are used because a composite material with acceptable cohesion characteristics is generally produced at relatively low production costs.
According to the prior art, these composite materials can be prepared in the following manner: the bundles of filaments are mixed with an aggregating resin, pitches and other additives and the mixture is placed in a mould in which it is formed by heating and the application of a pressure to produce a shaped semi-finished product.
The semi-finished product is then subjected to a first firing in a furnace at a temperature such as to bring about carbonization or pyrolysis of the resin.
As a result of this firing, the semi-finished product acquires a degree of porosity because of the loss of volatile material at the carbonization or pyrolysis temperatures.
The fired semi-finished product is then subjected to a second firing in the presence of silicon at a temperature such as to bring about fusion of the silicon and infiltration thereof into the pores of the semi-finished product.
The infiltration of the silicon increases the cohesion of the bundles of carbon filaments and, at the same time, the fused silicon reacts partially with the carbon of the semi-finished product in the conditions of the second firing, forming silicon carbides which have the effect of improving the cohesion characteristics of the material.
The composite material prepared by the method described above is often used in the production of brake and clutch components for vehicles, particularly for disk brakes, by virtue of its good characteristics of compression strength and resistance to the heat generated by friction, and to wear.
In spite of the above-mentioned good characteristics, disk brake disks have the serious disadvantage that any cracks or fractures which may form in them as a result of thermal and/or compression stresses tend to propagate rapidly throughout the structure of this material, bringing about its disintegration.
The use of known materials for a vehicle""s disk brake disks thus clearly leads to considerable risks to the user""s safety.
It is desirable to have a disk brake disk that has structural and functional characteristics such as to overcome the disadvantages mentioned with reference to the prior art.
A disk brake disk is described that resists disintegration from crack propagation. The disk brake disk of one embodiment of the invention comprises a braking band for cooperating with brake calipers in order to exert a braking effect on a vehicle, the braking band comprising a shaped body which extends about an axis of symmetry and is defined laterally by braking surfaces, the shaped body being made of composite material which can be produced by causing a mixture comprising bundles of filaments constituted substantially by carbon, arranged randomly and having dimensions no greater than 30 mm, to interact with silicon at a temperature high enough to bring about fusion of the silicon, characterized in that a plurality of reinforcing fibers is incorporated in the shaped body and extend around the shape of the shaped body in a manner such as to prevent the propagation of cracks.
The present invention is based upon the surprising discovery that the incorporation of reinforcing fibers in a mixture comprising bundles of randomly-arranged filaments within the shaped body of the disk brake disk produces a disk of composite material with a shaped body which has good cohesion characteristics and at the same time can prevent the propagation of cracks through the entire shape when the disk is in use.
The reinforcing fibers extend, in the disk structure of composite material according to one embodiment of the invention, around its entire shape.
In an alternative embodiment of the invention, the reinforcing fibers are provided only in some regions of the disk, for example, in the regions in which cracks arise, as well as in regions affected by the crack-propagation paths, these regions being easily identifiable on the basis of structural calculations, for example.
In the case of an axially symmetrical structure, such as a disk brake disk, structural calculations show that the crack-propagation paths have the greatest probability of being arranged radially relative to the shaped body, propagating from the inside of the disk towards the outside to the extent of causing the disk to suddenly fail.
In a disk brake disk according to the present invention, the propagation of cracks is therefore deterred by arranging the reinforcing fibers, for example, around annular portions of the disk of increasing size.
It is important that the reinforcing fibers have satisfactory characteristics of cohesion to the other components of the composite material constituting the disk to prevent the entire structure from disintegrating in use, even in the absence of cracks or fractures.
Moreover, the reinforcing fibers should be substantially inert in relation to the components of the composite material and should have an adequate ability to withstand the temperatures of pyrolysis and the infiltration of silicon in order to avoid their degradation during the preparation of the composite material.
In one specific embodiment the material of the reinforcing fibers is constituted by carbon fibers. In other embodiments of the invention other materials such as SiC, Si3N4, and TiC, as well as metals, for example, platinum, which can withstand the temperatures of the interaction with silicon, are used as the material of the reinforcing fibers.
The reinforcing fibers may be incorporated in the material in various ways. For example, the reinforcing fibers may be disposed in a plurality of bundles which are arranged in predefined directions.
These directions may be, for example, warp and weft directions, the bundles forming a fabric.
Alternatively, the reinforcing fibers may constitute a non-woven fabric, for example, a felt.
The reinforcing fibers may also constitute one or more layers within the shaped body of the disk.
The quantities of the components contained in the composite material shaped in the form of a disk may vary, as percentages by volume relative to the volume of the material, as follows:
bundles of filaments 40-70%, preferably 50-60%,
binder 5-30%, preferably 15-25%,
additives 0.5-20%, preferably 1-15%.
reinforcing fibers 4-30%, preferably 10-20%.